Firing new shots

Using lasers to trigger fusion could prove cheaper than other techniques

WHEN lasers were invented in the 1960s they were little more than an exotic curiosity—a solution looking for something to solve. Now that lasers are found in everything from telephone cables to DVD players to the humble laser pointer, they have become almost prosaic. Time, then, to rekindle a little glamour, by using lasers to bring about nuclear fusion, the great prize of energy research. Next month researchers will try to persuade the European Commission to pay for the biggest laser-fusion project yet.

Nuclear fusion is a compelling idea. The sun and stars are daily proof of the great energy released when light atomic nuclei join together. Lots of power could be produced using very little fuel. No greenhouse gases would be released. The fuel is abundant, the process safe and the waste, though radioactive, is far less noxious than the stuff left over from fission. Unfortunately, in spite of more than 50 years' work and billions of dollars of backing, no one has yet managed to get an Earth-bound fusion reaction to produce useful energy.

The idea of using lasers to trigger fusion has a long history. It is partly behind the National Ignition Facility at the Lawrence Livermore National Laboratory in California, due to be completed next year, and a slightly larger effort, called the Laser Mégajoule, in France that is a couple of years further off. Most of the work done by Livermore's 192 powerful lasers, focused on a spot just a millimetre or so across, will be nuclear-weapons research. But some of the time they will blast a pellet of nuclear fuel. The fuel will be made from two heavy forms of hydrogen, called deuterium and tritium. Zap it from all sides simultaneously with the lasers and, scientists hope, the pellet will implode and the hydrogen nuclei will fuse. They would form helium and a subatomic particle, called a neutron. The neutron would carry energy that could be converted into heat. In principle, this heat could be used, as in conventional power plants, to boil water and create steam that drives a turbine.

Taking aim

Laser fusion is the poor cousin of magnetic-confinement fusion. That technique uses a magnetic field to hold large volumes of the same isotopes of hydrogen inside a doughnut-shaped vessel, called a tokamak. The gas must be far hotter than the dense matter at the centre of the sun for fusion to happen. At such temperatures atoms are stripped down to their electrically charged components, so the helium created by fusion remains within the magnetic bottle. But the electrically neutral neutrons escape to the walls of the vessel where the energy they carry can be used to generate power.

Construction of the world's main demonstration power plant that will use magnetic confinement is under way at Cadarache in France. It is a joint project between America, most of the European Union, Japan, China, India, Russia and South Korea. Called the International Thermonuclear Experimental Reactor (ITER), it will cost some $5 billion to build, $5 billion to run for 20 years and a further $1 billion to decommission. The builders should have moved out by 2016.

Laser scientists want a slice of this action. Their bid to the European Union is to build an international laboratory, called “HiPER”, to run in parallel with the ITER machine. Bureaucrats in Brussels are interested. Canada and Russia have become involved and talks are under way with America, China, Japan and South Korea. The price tag is relatively cheap: a mere $1 billion.

HiPER would use a promising new “fast ignition” technique. This laser-fusion technique was achieved in 2001 by Ryosuke Kodama and colleagues at Osaka University in Japan. The standard approach, being pursued in America and France, works like a diesel engine by compressing the fuel until it ignites. This calls for the lasers to be very finely tuned and the fuel pellet to be perfectly smooth, so that the implosion is symmetrical and fusion occurs. The fast-ignition technique is more like a petrol engine: first the fuel is compressed and only then is it ignited by a second laser pulse—acting as a spark plug—that is fired through a hole in the pellet.

The upshot is that nuclear fusion can happen using rough-and-ready lasers and rough-and-ready fuel. Fast ignition also takes less powerful lasers, because the reaction rides on the energy contained in the pellet after the first pulse. So it is more efficient, too.

Japan's laser-fusion programme is like the proposed European one, but is smaller in scale. It does, however, have the advantage of already having started, so future upgrades will give a chance to compare results from the Japanese machine with the American and French results, which are expected in the next few years.

Lasers offer some obvious advantages over magnetic confinement. Because laser beams can run for long distances, the fuel does not have to be right next to the lasers that generate them. This means they can be easily shielded from the damaging neutrons. Fuel pellets can also be placed in the target site using conventional mechanical systems and these should easily be able to cope with the five mini-explosions a second needed to generate power.

There is one problem with nuclear fusion that lasers cannot solve, however: the price. HiPER may be budgeted to cost just one tenth as much as its magnetic cousin. But $1 billion is a lot of money all the same. Whether the benefits are worth such costs remains written in the stars.